A technique that authenticates communication data communicated on a network by a message authentication code (Message Authentication Code: MAC) is known (see, e.g., Patent Literature 1). In a technique disclosed in Patent Literature 1, each of communication apparatuses connected to the network increments a value which is stored in a corresponding counter, each time another communication apparatus transmits a main message.
A transmission communication apparatus transmits the main message and a MAC message that includes a MAC generated based on the main message and a value counted by the counter.
A reception communication apparatus determines whether the MAC, which is generated based on a received main message and the value of the counter stored in the reception communication apparatus, matches a MAC included in the received MAC message. With this configuration, the reception communication apparatus determines validity of the received main message.